This invention relates to an optical system. In particular, but not exclusively it relates to an optical system forming part of a thermal imager of the type comprising a telescope and a scanning system for scanning the field of view and directing radiation from the field of view onto a cryogenically cooled detector.
In the past, scanning systems used in such thermal imagers have employed a series of mirrors to give a compact design. Part of a typical imaging system is shown in FIG. 1 and comprises a scanning polygon which scans the field of view in a horizontal direction, a frame mirror which scans the field of view in a verical direction, and a relay system including a relay mirror which is, of necessity, operated off axis so that radiation may be directed ultimately to a cryogenically cooled detector. The relay mirror is used to image the entrance pupil onto the frame mirror to facilitate the scanning function.
However, the use of a spherical relay mirror, that is, a mirror having only one radius of curvature, introduces a degree of asymmetry due to the requirement to operate it in such an off axis manner, which introduces aberrations into the system. To avoid such aberrations it is possible to replace the spherical relay mirror with a mirror having two mutually orthogonal radii of curvature. Such a mirror is commonly called a toric mirror since its surface may be considered to be a portion of the inner surface of a torus. Due to the radius in the horizontal plane, the mirror is able to reflect radiation onto the line mirror without introducing substantial aberrations.
The use of a torical mirror does however have some disadvantages. Toric mirrors are expensive to manufacture due to their complex surface curvature. Further, they are commonly made by diamond turning techniques which result in very fine scratching of the reflective surface. These fine scratches act as an optical grating and thus introduce further aberrations and scatter.